Vertical lift doors are used in a variety of industries where structures require large doorways. For example: aircraft hangars require hangar doors large enough for an aircraft to pass through, factories often require large doorways for receiving materials and equipment as well as the exiting of finished products.
The type of vertical lift door for such applications may vary based on specific industry requirements, structural support capacity, or the intended environment. Generally, two types of vertical lift doors are common. First, a hoist up fabric door and second, a metal clad paneled vertical lift door. Regardless of the type of vertical lift door, the doors are typically constructed with multiple panels spanning the width of the building opening. Additionally, each panel may weigh 100 lbs or more, with many over 1,000 lbs.
In order to lift and lower these large vertical lift doors, a lift mechanism usually in the form of a power driven motor is connected through a cable or similar retractable tension bearing member to each side of the door. The cable may feed through pulleys or brackets on separate panels to create uniform lifting where each panel's vertical travel is relative to the other panels.
The heavy weight of the door and its inherent exposure to adverse environmental conditions lead to substantial lift cable and lift mechanism wear. In the event of a catastrophic failure occurring as a result of a break in the lift cable or failure of the lift mechanism, significant damage to the vertical lift door or the building structure may occur, as well as possible injury to individuals or damage to goods in the vicinity of the failed door.
To obviate the effects of such catastrophic failures, safety catch systems have been devised to prevent free fall of the vertical lift doors. One form of fall arrestor is disclosed in U.S. Pat. No. 4,368,770 to Ulfhielm, which utilizes a vertical guide bar extending along an edge of the door and having a U-shaped cross section. A compression spring is normally compressed as a result of the door weight and tension applied by a lift cable. When the lift mechanism fails and the door begins to fall, the spring expands and actuates a pair of latching dogs which extend away from each other against opposite sides of the U-shaped guide bar. However, the latching dogs extending in opposite directions from each other against the opposed sides of the U-shaped guide bar may fail to provide a reliably strong binding effect in the event of a catastrophic failure of the cable or cable lift apparatus. The forces applied by the dogs to opposing sides of the guide rail may result in deformation of the U-shaped guide bar and significant structural damage to the vertical doorframe.
Another form of vertical door fall arrestor is disclosed in U.S. Pat. No. 6,553,716 to Bruns which discloses a safety catch system comprising a vertical guide rail configured between a pair of latching dogs that are positioned free of the guide rail through a biasing mechanism and selectively actuated by a control arm connected to a lift cable and lift mechanism. In the event of a catastrophic failure of the cable mechanism, a torsion spring surrounding an axle connected at one end to the latching dogs and at the other end to a stay/release mechanism rotates the latching dogs into contact with the guide rail. Due to the infrequent activation of the biasing mechanism and its exposure to adverse environmental conditions, the system is prone to corrosion and contamination preventing reliable activation and binding to the guide rail in the event of a catastrophic failure. Furthermore, a direct drive connection such as a torsion spring mounted on an axle which the spring must rotate in a catastrophic failure requires greater torque than a compression spring utilizing a cam mechanism to pivot the latching dogs.